Recycling A/C water?

The other day I found out to my dismay that the balcony underneath my air conditioner at home leaks water into the living room below it. Instead of sealing the balcony (as I should...) I temporarily solved the problem by placing a baby bath tub underneath. Yep, I'm lazy.

Two days later, I noticed that the baby bath tub is almost full. So, I took it down and watered the lawn with it. It made me think whether the total water condensed in Israel's A/C's is sufficient to be of any real use. If you live in Houston, you probably noticed that the A/C's condense a lot of water, however, since it rains profusely there, this water is not going to make any difference. Israel, on the other hand, is using all its available fresh water and has began desalination. Watering gardens with condensed water could help. The question is by how much.

Living near Jerusalem, I actually don't need too much A/C, perhaps two weeks over the summer. Namely, 7 baby bathtubs worth of water, or about 150 kg, or 0.15 of a cubic meter. Since I pay my municipality about 1$ per cubic meter I could save about 15 cents by recycling the water over the summer. Not much, to say the least.

What about someone living along the coast, such as in Tel-Aviv? Over there, the high humidity implies two things. First, it is unpleasant all summer, so the A/C is constantly on for a few months. In addition, more water is condensed by the A/C per unit time, because of the higher water content. Let's suppose that during the day, the typical outdoor conditions are 30°C with 70% humidity, while the indoor A/C conditions are about 23 and 60% humidity. Using a simple water content calculator, one finds that we're cooling the temperature by 7 degs and taking out of it about 10 g of water per kg of air. Using the heat capacity of air, and the latent heat of water, implies that we need about (1kg)*(7°K)*(1000J kg-1°K-1) = 7,000J to cool a kg of air and 10gr*2272J/g=22,000J to condense the water in the kg of air. In other words, for every unit of energy used by the A/C, 25% are used to cool the air, and 75% to dehumidify it, at least, if you're cooling fresh outdoor air. After the initial dehumidifcation of the closed area, the A/C would start spending more of its energy on just cooling. In desert areas, on the other hand, there is little water to begin with and a smaller fraction would be used for dehumidification also when cooling fresh outdoor air.

If you're a private person in a humid place like Tel-Aviv or Houston, you would be operating an A/C. For every kWhr of electricity, you typically get 3.5 times as much "cooling energy" because of the large efficiency of A/C which are heat engines. Thus, 1kWhr, which would be used by a typical one room A/C of say 12000 BTU/hr, operating for an hr, would give you 3600*1000J. If half this energy is going to dehumidify the air, then you would condense about 0.5*3600*1000J/(2272 J/gr) = 800 gr of water. Of course, the ratio between the cost of electricity and water is different from place to place. In Israel, a kWhr would cost you somewhat more than 10 cents, while a cubic meter, costs about 1$. This implies that by recycling, you could save just less than 1% on your A/C electricity bill. Still not much. If you spend a few hundred $ per summer on A/C-ing, you would save a few bucks, enough to buy a simple dripper line. After a few years, you would save enough money to buy yourself a simple dinner.

What about the national level? Over summer, Israel spends about 150,000 kWhr per year on A/C (e.g., this document). If half the energy is used for dehumidification, Israel condenses 0.5 * 150,000,000 Whr * 3,600,000 J/Whr / (2272 J/gr) / (1,000,000 gr/m3) = 0.1 million m3 of water a year. For comparison, Israel annually uses 1,800 million m3, i.e., A/C condensed water is literally a drop in the ocean. Using condensed water on a large scale is simply not practical. Oh well, another idea down the drain...

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Comments (5)

Marc Graves

It's this kind of out of the box thinking in relation to water use that will become more important than ever. Dwindling fresh water suppliies worldwide will be one the most serious problems we will face in the near future. It won't be long until water is more expensive than oil. We can live without the oil...living without water is not an option!

Jan 30, 2009

Anonymous (not verified)

Hi I live in Dubai. One split unit a/c in the height of summer produces between 90- 120 litres of water a day, especially when the humidity is high. I use only this to water 40 plants in pots, in my courtyard, trees shrubs and annuals, the plants look beautiful and really create a haven from the dry desert, its also important to have open weave shade as that help to let through enough light for the plants but cools down the courtyard, so less stress on the plants. In a very humid place like this recycling a/c water makes sense.

Jun 30, 2009

shaviv

When the water is available and diverting it to the pots is cheap, then sure, it is a wise and nice thing to do. However, the cost to desalinate 100 liters is about 5 cents!

Anyway, I am lucky myself to live mostly above the inversion layer, so the air is dryer. In the days I do need A/C, I get about 5 liters from the whole house. Not a lot at all.

Recycling A/C condensed water is not practical (or even doable) on a large scale. Not the way things are now. But if we would have centralized A/C stations that would provide A/C to large parts of the city (same as we do with hot watter) then recycling would become easy.
And it really isn't such a crazy idea. Just think about it - it wasn't long ago that every household heated their own watter (a lot of them still do) which is less practical and less economical than using centralized heating stations.

Feb 11, 2009

Anonymous (not verified)

I live in Houston, usually very humid. We normally run a/c 6 months. I recently got 3, 50 gallon rain barrels. I piped the a/c drip lines to the gutters, collecting all the drip water. All 3 barrels were full in a week. The barrels have integrated hoses to attach regular watering hoses to. We usually get an inch of rain a week. We haven't had 3 inches in 18 months. We have water rationing. I use the a/c drip water to water my trees. As they gradually die, branches fall off randomly. The first one missed my car by a foot. If a tree died it would cost $500-1000 to remove. Sometimes its not the cost of the water.